Waste water streams from industrial plants, including petroleum refineries, chemical plants, pulp and paper plants, mining operations, electroplating operations, and food processing plants, can contain offensive substances such as cyanide, sulfides, sulfites, thiosulfates, mercaptans, and disulfides that tend to increase the chemical oxygen demand (COD) of the waste water streams. Examples of these waste water streams in petroleum refineries include sourwater, sourwater stripper bottoms, and spent caustics. The Environmental Protection Agency (EPA) and various local agencies have placed limits on the allowable levels of these substances in industrial waste water effluent streams.
The conventional methods for waste water treatment include incineration, biological oxidation, and chemical oxidation using H.sub.2 O.sub.2, Cl.sub.2, NaOCl, ClO.sub.2, and KMnO.sub.4. However, the concentration of the cyanide or other material in the waste water may be too low to treat economically using conventional means, but still too high to meet effluent limitations.
It is well known that sulfides in waste water, including sourwater stripper bottoms or foul water, can be oxidized using air to reduce the chemical oxygen demand of the waste water. These air oxidization processes are commonly practiced in petroleum refineries. In these air oxidation processes, the sulfides are oxidized to thiosulfate, and finally sulfate, as is shown in the following representation: EQU 2S.sup.= +2O.sub.2 +H.sub.2 O.fwdarw.S.sub.2 O.sub.3.sup.= +2OH.sup.-( 1) EQU S.sub.2 O.sub.3.sup.= +2O.sub.2 +H.sub.2 O.fwdarw.2SO.sub.4.sup.= +2H.sup.+( 2)
As reported by Beychok ("Aqueous Wastes from Petroleum and Petrochemical Plants," pp 208-211, John Wiley, 1967), Abegg et. al. ("A Plant for Oxidation of Sulfide Containing Refinery Waste by Air", Ardol Kohle Erdgas Petrochemie, September 1962) noted that the reaction rate of sulfide to thiosulfate in the presence of air, as represented by equation (1) above, is relatively rapid. Unfortunately, the reaction rate of thiosulfate to sulfate, as represented by equation (2) above, is extremely low, so that in an air oxidation process, most of the sulfides are converted to thiosulfate. A second, more severe process is required to oxidize the thiosulfate to sulfide. Based on Abegg's data, Beychok observed that, "To oxidize 34% of the sulfides to sulfates requires a tenfold increase in tower volume compared with units that oxidize the sulfides to thiosulfate." Thus, a catalyst is required to convert sulfides and thiosulfate efficiently to sulfate.
Copper is an effective catalyst for oxidation of sulfides and thiosulfate. Beychok also observed that by use of homogeneous CuCl.sub.2 catalyst, sulfides can be converted completely to sulfates. Continuous addition of a homogeneous catalyst to the treatment system is undesirable because of the chemical and operating costs, and most importantly, pollution of the treated water by copper.
In developing water treatment processes, particular concern is directed to processes which do not leave residues in the treated stream. Residues can cause additional disposal problems. Materials consumption and cost is also an important factor; thus, it is important to avoid processes which require replenishing the supply of costly catalyst and reagent.